The invention relates to a process for compounding heat-vulcanizable silicone compositions.
A heat-vulcanizable silicone composition comprises a high viscosity silicone polymer, an inorganic reinforcing filler and various additives that aid processing or impart desired final properties to the composition. A vulcanizing agent can be added and the composition heat-cured to fabricate silicone rubber moldings such as gaskets, medical tubing and computer keypads.
Typically, the heat-vulcanizable silicone composition is produced by kneading a high-viscosity polydiorganosiloxane, the inorganic filler and additives by means of a batch kneading machine such as a high intensity Banbury mixer or a low intensity double arm dough mixer. In this process, polydiorganosiloxane, inorganic filler and treating agents are batch mixed until desired properties are obtained. This process requires long residence times and large amounts of energy. Non-homogeneous shear and extensional stresses across a commercial sized batch can result in non-uniform size distribution of filler that results in variations in properties. Batches processed at different times may be characterized by different physical properties. The batch process is labor, energy and capital intensive and produces materials of only marginal consistency.
Wacker-Chemie GMBH, EP 0 570 387 B1, discloses mixing, homogenizing and degassing diorganopolysiloxanes and finely divided silicon dioxide and, optionally, other components in an oscillating single-shaft pilgrim step kneader. However, while single-screw extruders can be used for melting and pressurization operations, the capability of a single-screw extruder to mix materials of different bulk densities is limited. High length-to-diameter ratio single shaft extruders are required to compound high levels of a low bulk density powder, such as fumed silica, into a high viscosity matrix, such as a diorganopolysiloxane. This is particularly so for raw, untreated fumed silica due to the difficulty in incorporating the silica into the diorganopolysiloxane. However, longer extruders require higher torque and produce higher temperatures. The higher temperatures and the dissipation of viscous heat over longer periods of time result in crosslinking and degradation of material.
In Kasahara et al., U.S. Pat. No. 5,198,171, a preconcentrate of polydiorganosiloxane, inorganic filler and treating agents is formed by a high speed mechanical shearing mixer. The resulting premix is further compounded in a same-direction double screw extruder. The premix is formed in a first step wherein a diorganopolysiloxane having a viscosity at 25xc2x0 C. of 1xc3x97105 cP or more, an inorganic filler and a treating agent are mixed in a high-speed mechanical shearing machine to provide a flowable particulate mixture in which each ingredient is present in a substantially uniform, finely dispersed state. The flowable particulate mixture is then fed at a constant feed rate into a kneading and extruding machine that has two screws rotating in the same direction.
In Hamada et al., U.S. Pat. No. 5,409,978, a preconcentrate of polydiorganosiloxane, inorganic filler and treating agents is formed at a temperature in the range of about 200xc2x0 C. to 300xc2x0 C. in a co-rotating, continuous double screw extruder. The preconcentrate is then compounded and heat treated at 150xc2x0 C. to 300xc2x0 C. in a counter-rotating, double screw extruder.
In these processes, the fumed silica filler and silicone gum must be compounded, either batch or continuously, into a free-flowing particulate before it can be fed continuously into a compounding extruder at a reasonably high rate. Variations in feed, temperature, residence time, etc. that may be introduced up-stream in a compounding process are propagated through any down-stream step. Hence a pre-densification step causes an increase of the variability of the overall process leading, particularly in the case of batch preparation of pre-mix, to materials with inconsistent properties.
There is a need for a low cost process that continuously and consistently produces a full range of both low viscosity and high viscosity silicone elastomers from filler, additive and polymer. There is a need for a continuous, robust extrusion process that does not require a preconcentrate of filler and polymer feed to manufacture heat-vulcanizable silicone compositions. Further, there is a need for an improved single step compounding process.
The invention provides a process that compounds high levels of inorganic filler, processing fluid and silicone polymer into homogeneous filled silicone compositions with requisite reinforcing properties and levels of volatiles. The process can be conducted continuously without forming a preconcentrate of filler and polymer. The process comprises mixing a filler with a processing fluid at a first location of a unitary continuous compounding apparatus prior to addition of a silicone polymer. The filler is then mixed with the silicone polymer, which is fed into the compounding apparatus at a location down-stream from the first location.
In another aspect, the invention relates to a process for compounding silica filled silicone utilizing a co-rotating, double screw continuous compounding extruder. In the process, fumed silica is continuously fed to the co-rotating, intermeshing double screw extruder at a first location, prior to addition of a silicone polymer. The fumed silica is compounded with a processing fluid or combination of processing fluid and treating agent as the fumed silica advances to a second location. A silicone polymer is fed into the compounding apparatus at the second location and the fumed silica is compounded with the silicone polymer by means of the extruder as the fumed silica and silicone polymer advance through the extruder from the second location.
In another aspect, the invention relates to a processable silicone polymer filler composition comprising a processing fluid and a filler. The processing fluid is combined with the filler in a range of from about 0.1 to about 100 parts fluid per 100 parts by weight of filler. The invention also relates to a processable silicone polymer composition, comprising a processing fluid, a filler and a silicone polymer. The processing fluid is combined with the filler in a range of from about 0.1 to about 100 parts fluid per 100 parts by weight of filler and the filler is combined with the silicone polymer in a range of about 5 to about 200 parts filler per 100 parts by weight of polymer.
In still another aspect, the invention relates to a compounding apparatus, comprising a barrel encompassing at least one shaft with screw flights arranged in sections along the shaft according to a type of materials processing. The sections comprise (1) a first conveying section, (2) a second kneading section, and (3) a third devolatilization section.